[en] Two reasons, in my opinion, explain why people might want to blog anonymously: (a) to prevent people they know from reading what they write on their blog; (b) to prevent unknown people who read the blog from tracking them down. In both cases, there is a desire to create some kind of barrier between online and offline. In the first case, the aim is to prevent offline from penetrating online. In the second one, it is to prevent online from penetrating offline.

I think people who "go anonymous" for the first reason are those who are at risk of losing their jobs, falling out with family and friends, or at best, spend a few embarrassing moments if they are "outed". I personally think it's a pretty risky thing to do. On the other hand, I think the second reason can make sense, and even be a sensible choice in some cases -- for example, in the case of a lawyer who would not want to be contacted for professional reasons by people who know him through his weblog.